Glaucoma is the second leading cause of blindness worldwide. The clinical diagnosis of glaucoma is primarily based on characteristic patterns of visual field toss, progressive retinal nerve fiber layer (RNFL) thinning and optic nerve head (ONH) changes. Historically, the focus of glaucoma detection has been on the RNFL thickness. Several devices employing different imaging modalities, e.g., confocal laser ophthalmoscopy, scanning laser polarimetry and optical coherence tomography (OCT), produce measurements of these thicknesses. Raw OCT data, however, provides local measurements of scattering properties of the tissue and may therefore produce additional measures of the RNFL.
U.S. Patent Publication No. 2010/0208201 described segmenting the intensity data and assigning a single representative intensity to a segmented portion, and displaying a 2-D image of the assigned intensity. With respect to glaucoma diagnosis, the reflectivity of the RNFL has been shown to decrease in glaucoma (as described in Van der Schoot J, et al. IOVS 2010; 51:ARVO E-Abstract 212; Vermeer K A, et at, IOVS 2011; 52:ARVO E-Abstract 3666), especially in its early stages. Segmentation procedures were previously described in U.S. Pat. No. 7,782,464 to identify the boundaries between anatomical layers.
However, none of such described systems, methods and procedures describe a way to normalize the data to take into account instrument errors, ocular opacities, etc
Several techniques are used for depth-resolved imaging of scattering media, such as confocal microscopy and/or optical coherence tomography (OCT). In these techniques, incident light travels through the media, interacts with the media (e.g., an anatomical sample), and is collected by one or more detectors. The interaction of the light and the media can be complex, because interaction does not only take place at a single depth. Instead, the incident bundle generally interacts with many and/or all layers it passes through, scatters at some depth and the scattered beam again interacts with the media until it arrives at the detector.
For example, with OCT techniques, the sample is probed by a coherent light source and the depth-resolved backscatter signal intensity is recorded. The exemplary OCT techniques can be implemented in many ways, with fixed or moving reference mirrors, with spectrometers or swept-source systems, etc. In such cases, however, the OCT signal is generally dependent on energy of the backscattered beam that reaches the detector. Many of these measurements along a line are then combined to produce an image, as shown in FIG. 1.
In an exemplary OCT image, the intensity that is measured from a certain depth can be gray scale coded, where white can indicate a strong signal, and black—a weak or no signal. The OCT beam is generally incident from above on the tissue. Unfortunately, these images likely do not reflect the physical or optical properties of the tissue. Instead, they only illustrate the result of the complex interaction, which can mean that the same tissue may appear differently (i.e., with different signal intensity, illustrated by the different signal intensity of the RPE at the locations indicated by arrows 20 in FIG. 1) at different locations, determined by how the surrounding tissue is structured. This is because the exemplary OCT signal depends not only on the optical properties of the media at some depth, which result in the backscattered signal. Instead, the exemplary OCT signal is also dependent on the strength of the incident beam at that location, which is affected by the media it passes through first. In addition, the resulting backscattered beam again has to pass through some part of the media before it reaches the detector and is therefore further attenuated. This can result in artifacts that are frequently observed in OCT images. One example is the shading of blood vessels. Because the blood vessels cause a large reduction of the intensity of the incident light beam, the scatter intensity at deeper locations is largely reduced and is further attenuated on the way back to the detector. This can result in apparent gaps of underlying tissue, which clearly does not mimic the tissues structure (see FIG. 1, arrows 10). Another artifact is the very dim appearance of the choroid and sclera, both scattering tissue types, due to the attenuation of the incident light beam in other highly scattering layers, especially a retinal nerve fiber layer (RNFL) and the retinal pigment epithelium (RPE). Yet another artifact is the reduced intensity in the image in case of floaters or media opacities (e.g. in the cornea, the lens or the vitreous), which attenuates the power of the incident beam.
Further, because the measurements are the result of this complex interaction, the signal strength corresponding to a single depth measurement does not directly represent a physical or optical property of the medium at that depth. Instead, only morphological features of the measurements, often visualized in an image, are evaluated. However, these morphological features also depend on the signal strength and are therefore not always clearly defined in OCT images.
According to an exemplary embodiment of the present disclosure, it is possible to determine physical and/or optical properties of the medium from the measurements. For this determination, information of other, deeper locations can be included in the reconstruction process. An example of such a reconstruction is the determination of attenuation coefficients from OCT data. In this case, OCT data from both nearer and deeper locations are used to determine, iteratively, the local scattering intensity and the local attenuation coefficient.
In ophthalmology, OCT procedures have been used to image the retina for a number of years. Typically, a measurement is defined as an A-line, which contains depth-resolved backscatter data at a single transverse location of the retina. By using scanning optics, many A-lines are recorded along a transverse path, e.g. a straight line or a circle. In the past few years, it has gained in popularity due to increased scanning speeds, which facilitates an acquisition of volumetric three-dimensional (3D) scans by performing a raster-scan across the retina.
Not all data in an OCT scan is useful for the clinical task at hand. For example, in glaucoma, the RNFL can be the tissue layer that is of most interest. Segmentation procedures can be employed to segment the NFL in OCT data For compliance with conventional tests and because of easy interpretation, a segmented OCT scan can then be reduced to one RNFL thickness measurement for each A-line in the data set. In case of a two-dimensional (2D) data set, such as a circular scan around the papilla, this can result in a plot of the angle of the circle against the thickness at that location. Such plot can be called a TSNIT-plot. In a case of a 3D data set, such as a raster-scan of the peripappilary area, the result can be a thickness map, graphically showing the thickness of the RNFL at all scanned locations.
When reducing the segmented OCT data to these thicknesses (e.g., of a single tissue or multiple tissues) or distance (from one boundary of a tissue type to another boundary of the same or a different tissue type), the OCT data itself is not used. The produced data thus likely provide no information about the underlying tissue types. In case of glaucoma, the RNFL is known to deteriorate, tissue is lost and therefore the thickness of the RNFL decreases. However, the backscattering properties of the deteriorating RNFL may be different than that of healthy RNFL. Simply processing the absolute measurements will produce unreliable results. For example, media opacities may result in lower measured backscattering, which is not due to the measured tissue itself. Therefore, these measurements must be normalized by calculating the ratio of the measured backscatter to the backscatter of an unaffected structure with uniform scattering properties, such as the retinal pigment epithelium (RPE).
Accordingly, there is a need to address at least some of the deficiencies described herein above.